Valve for fire extinguishing system of gas distribution type

ABSTRACT

A line or directional valve of the type that is adapted for use in fire extinguishing systems of the gas dissemination type is provided, employing a moveable piston adapted to be unseated from a valve seat, upon delivery of fluid for opening thereof, wherein the forces of the valve piston against the valve seat are reduced and controlled by utilizing inlet fluid pressure that is operative against the piston in such a way as to reduce the forces from inlet fluid pressure on the piston to a desired net or resultant force. A vent is also provided for facilitating keeping the piston open after its initial opening.

BACKGROUND OF THE INVENTION

In cylinder valves of prior art types, when the valves are subjected tohigh inlet pressures especially in valves of larger diametralflowthrough dimensions, the forces that result from high pressuresworking surface areas of the piston are transmitted to the valve seats,which can become crushed leading to early failure.

SUMMARY OF THE INVENTION

The present invention is directed toward providing an offsettingpressure against a valve piston, that offsets a force component of theinlet fluid pressure on the piston, such that the resultant closingforce on the piston that urges it against the valve seat is at atolerable, generally predetermined level. This permits enlargement ofpiston bore size, for more rapid flowthrough of gas, and more rapidextinguishing of fire, without early damage and failure of the valveseat. A novel venting arrangement is provided for assuring that thepiston remains open during use.

Accordingly, it is a primary object of this invention to provide a novelline valve that utilizes inlet fluid pressure in such a way as to offseta force component of the inlet fluid pressure that urges the pistontoward the seated position, whereby there is a reduction in the force ofthe piston on the valve seat.

It is another object of this invention to provide a novel valvearrangement permitting larger flowthrough cross-section.

It is a further object of this invention to provide a novel ventingarrangement for maintaining the piston in an open condition in a line ordirectional valve.

Other objects and advantages of the present invention will be readilyapparent to those skilled in the art from a reading of the followingbrief descriptions of the drawing figures, detailed descriptions of thepreferred embodiments, and the appended claims.

BRIEF DESCRIPTIONS OF THE DRAWING FIGURES

FIG. 1 is a schematic view of the fluid distribution system, wherebyfluid may be delivered from a container, through a container or cylindervalve, through fluid conduit lines to a plurality of line or directionalvalves, to dissemination points in rooms to have the protection of fireextinguishing equipment, with sensing devices being illustrated forsolenoid valves that control the line or directional valves and thecylinder valves.

FIG. 2 is a longitudinal sectional view through a line or directionalvalve of the type illustrated in FIG. 1.

FIG. 3 is a longitudinal sectional view, taken through amanually-operable valve operator of the type used for operating acylinder valve, of FIG. 1.

FIG. 4 is a longitudinal sectional view taken through a container valveor cylinder valve of the type illustrated in the schematic of FIG. 1.

FIG. 5 is a longitudinal sectional view, taken through a vent valve forthe cylinder valve illustrated in FIG. 4, for keeping the piston cavityto the rear of the piston illustrated in FIG. 4 vented, once the pistonhas been opened.

FIG. 6 is an illustration of a tool of the type adapted for manual usewith the vent valve of FIG. 5, to keep the piston thereof in an unseatedcondition for continued venting after re-seating of the piston of thecylinder valve.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Referring now to drawings in detail, reference is first made to FIG. 1,wherein there is illustrated a system generally designated by thenumeral 10, for providing a preferably gaseous fire extinguishing fluidto rooms or zones 11, 12, 13, 14, or the like. This system 10 is adaptedto provide free extinguishing to rooms in which generally valuablecomponents, such as computer equipment 15 are stored, to rapidlyextinguish fires in those rooms without damage to the sensitiveequipment used therein. It will be apparent that other uses may be madeof the systems 10 of the present invention, as well as variations of theuses, but that use for computer rooms is set forth herein as an exampleonly.

The type of fluid utilized is a fluid that when compressed and deliveredin cylinder fashion is generally a liquid, but which, by the time itreaches its dispersing point for distribution into the zone of a fire,it is dispersed as a gas that need only be disseminated through a roomin low percentages by volume, but which is effective to extinguishflame, even at low volume percentage distribution (one percent to threepercent). A fire extinguishing fluid suitable for use with the system ofthe present invention is DuPont Halon 1301, for example.

The fire extinguishing fluid is normally quite expensive, and thereforethe present system has been developed whereby a single container 16 ofthe compressed fluid (generally liquid when compressed) may be provided,for servicing a plurality of protected zones such as the rooms 11through 14, by appropriate piping and valving. It will further beunderstood that this type of system may utilize a plurality ofcontainers or cylinders 16, which feed into a manifold that in turn mayfeed one or several directional valves for distribution into one or morerooms simultaneously, as desired.

A cylinder valve 17 is provided, mounted on top of the cylinder or othersuitable container 16, and a heat sensor, smoke sensor, or sensor ofother phenomena, 18 is provided in each of the rooms 11 through 14,preferably actuated by a suitable electric power source 20, foractuating a solenoid switch 21 by passing current through a suitableline 22, when the temperature in a given room, such as that 11 reaches apredetermined level. It will be apparent that each of the other rooms 12through 14 are equipped with similar sensing mechanisms illustrated, butnot specifically described herein, in the interest of avoidingduplication. It will also be apparent that the sensing means 18 may takeon any of various forms, and a plurality of such sensing means may beutilized in any given zone or room 11, as needed to adequately protectpersonnel or equipment or both in the room.

A mechanical actuator or operator 23 is provided for the container valveor cylinder valve 17, with the operator 17 being manually actuableeither by depressing a plunger or pulling a cable, or the like, to ventthe valve 17 to atmosphere through the operator 23 upon eitherdepressing the plunger 25 or tensioning a pullcable 26, as will befurther described hereinafter.

Upon opening the valve 17 by either electrical sensing followed byoperation of the solenoid 21, or by mechanical, pneumatic or any othertype of actuation of the operator 23, the valve 17 opens, to permitpassage of fluid into a distribution line 27, for delivery throughdistribution lines 28, 30, etc., to suitable line valves or directionalvalves 31.

The directional valve 31 may pass fluid delivered from the line 28through a line 34 to a distribution head or point 35, from which pointthe fire extinguishing gas is disseminated into the room 11.

The valve 31 is provided with a fluid line 36, that communicates withthe line 28 for receiving fluid, and with a fluid line 37 thatcommunicates with the valve 31 for deliveringvalve-opening-fluid-pressure to the valve 31, where needed to open thevalve, upon the valve 31 being actuated or operated either by operationor actuation of the manual valve operator 38, or by operation oractuation of the electrically operated solenoid valve operator 40. Themanual valve 38 may be operated by manual actuation of the push orplunger member 41, or by pulling of the cable member 42, orpneumatically, as desired, to open the line 36 to the line 37, foropening the valve 31. In the alternative, or even simultaneously, thesolenoid valve 40 may be actuated to open the line 36 to the line 37, bymeans of one or more heat detection, smoke detection, or other suitablesensors 43 in the room 11, powered by a suitable power source 44, suchthat, when the temperature or other phenomena in the room 11 reaches thepredetermined level, not only will the sensor 18 be actuated, but alsothe sensor 43, for operating the solenoid that controls the solenoidvalve 40.

It will be apparent that each of the lines 30, 32, and 33 are providedwith similar valves, piping, and controls to those 31, 36, 37, 38 and 40heretofore described, and that the rooms 12 through 14 are also providedwith suitable sensors, similar to those 43, as well as distribution ordispersion points, as needed.

It will be apparent from the foregoing that should a fire commence inone or more of the rooms 11 through 14, a sensor 18 (generallyelectrically operative) will open the valve 17 and make gaseous fluidavailable to each of the directional valves or line valves like thevalve 31. Substantially simultaneously, the sensor 43 will open thedirectional valve for delivery of gas through the distribution ordispersion point in the room in which the fire is located. It will alsobe apparent that the valves 17 and 31 may be manually actuated ifdesired, based upon a manually actuating valve design, or moreimportant, should the electronic sensing mechanism fail in the fire, thesystem may still be utilized by manual actuation. It will further beunderstood that the sensors, instead of being electrically operative,could be of the type that produces a pressure signal, and in that casethe operators 23 or 38 may be of the pressure responsive type. It willfurther be apparent that the sensors 18 and 43 could be connected inseries as a precaution against accidental gaseous discharge so that thevalves controlled thereby could not be other than simultaneously opened.

Referring now to FIG. 2 in detail, the line valve or directional valve31 will be described in greater detail. At this point the fluid isdelivered as a gas to the inlet 50 of the valve, generally at a desiredpredetermined pressure, for exposing the generally cylindricallyconfigured piston 51 longitudinally moveably disposed within the casing52 thereof to the desired pressure. The pressure is operative againstthe end 53 of the piston 51, as well as against the closed end 54thereof, an outer portion of which is illustrated in FIG. 2 in seatedengagement against an annular rubber, plastic or the like valve seat 55.A suitable ring seal of rubber or like construction 56 seals the outersurface 57 of the piston 51 relative to the member 50. A cylindricallywound spring 58 seated in an annular slot 60, and in engagement againsta transverse surface 61 of the piston 51, urges the piston 51 towardseated engagement against the valve seat 55.

The gaseous fluid received at the inlet 62 is free to pass up thecommunication port 63, to the line 36, and when permitted by openingeither the manual operator 38 or the electrically operated solenoidoperator 40, the inlet pressure 62 is delivered to the line 37, throughthe inlet 64, to bring the transverse second opening-pressure-operativesurface portions 65 into contact with operating fluid pressure, in adirection opposite to the direction of force application of theoperating fluid against the first opening-pressure-operative surfaceportions 53 and 54, for example, and in a direction toward compressionof the cylindrically wound compression spring 58, whereby, under theforce of fluid provided against surface portions 65, the piston 51 willmove from the full line position illustrated in FIG. 2 to the phantomline position illustrated therefor, bringing the surface 66 toward, oragainst the surface 67.

A suitable vent 68 is provided, for permitting leftward movement of thepiston 51 between the full line and the phantom positions illustrated inFIG. 2, in order that air entrapped in the zone 70 will not prevent theopening movement of the piston 51. It will also be apparent that asuitable ring type seal 71 is provided, preventing leakage of gaspressure between the members 52 and 51 along sliding surface zone 72.

A plurality of openings 73 are provided through the cylindrical wall 74of the piston 51, but ahead of the valve seat 55, as illustrated in FIG.2, in order that incoming fluid pressure will exist in the zone 74 thatis cut out of the casing 52, for exposure of the transverse thirdsurface portions 75 to opening fluid pressure, while the piston 51 isseated against the valve seat 55.

In the static condition of the valve 31, with fluid pressure that isdelivered at the point 62 also being delivered for exposure of thesurface 75 thereto, and with the surface 75 facing in an oppositedirection to the surfaces 54 and 53, it will be noted that, with thevalve operators 38 and 40 closed, such that no opening fluid pressure isdelivered to the line 37, and with the closed end portion 54 of thepiston 51 in seated engagement against the valve seat 55, the samepressure that is delivered against the first surface portions 53 and 54,to tend to urge the piston 51 toward the valve seat 55, is alsodelivered to the surfaces 75. This provides a counter-pressure force,that operates in a direction that, by itself, would tend to urge thepiston 51 open, with its closed end portion 54 away from the valve seat55, but that is of such a selected area size that it serves to offsetsome of the force with which the piston 51 would otherwise be urgedagainst the valve seat 55. Accordingly, the size of the annular surfacearea 75 that operates to at least partially reduce the net longitudinalpressure on the piston 51, is carefully selected to offset as much aspossible unnecessary seating forces of the closed end portion 54 of thepiston 51 against the valve seat 55. For example only, but not by way oflimitation, if the inlet fluid pressure at the location 62 is 360p.s.i., the outer diameter of the outer piston wall portion 76, and theinner diameter 77 that defines the surface area 75 may be selected suchthat the ultimate net force of the piston 51 against the valve seat 55does not exceed 1,000 pounds, should the same be desired, when thepiston 51 is in the static condition illustrated in full lines in FIG.2. Also, if desired, any forces provided by the compression spring 58may be taken into account in selecting the desired size for the surfacearea 75. It will therefore be seen that dimensional selections for thevarious components may almost keep in balance the various forces actingon the piston 51, but with there always being a net resultant force thatkeeps the closed end portion 54 of the piston 51 disposed against thevalve seat 55, when the valve 31 is to be closed as illustrated in fulllines in FIG. 2.

When the lines 36 and 37 are opened to each other, such that the piston51 is moved to the phantom line position illustrated in FIG. 2, it willbe noted that a vent opening 80 through the cylinder wall 74,communicates the interior 81 of the piston with the inlet 64, to permitventing thereof, so that the piston 51 will not return to its closedcondition while it is desired that the valve 31 be open.

When the valve 31 is open, with the piston 51 in the phantom positionillustrated in FIG. 2, it will be apparent that the gas is deliveredthrough the openings 73, around the closed end 54 of the piston 51,through the hole 82 in the valve seat 55, and through the line 34, to adispersion point 35. It will therefore be apparent that, even when avalve seat 55 is constructed of rubber, plastic, or other suitable,generally softer materials than the materials in construction of theclosed end portion 54 of the piston 51, and even when the valve 31 issubjected to very high pressures, undue forces will not be imposed onthe valve seat 55 that may otherwise tend to crush them, but that thepressures on the valve seat 55 may be carefully controlled due to theuse of counter-pressure on the surfaces 75.

The solenoid valve 40 may be of any suitable type, such as will open thelines 36 and 37 to communication with each other, upon the sensor 43 (ifelectrical) causing to complete the circuit between the power source 44and ground 39. In the alternative, any other suitable circuitry may, ofcourse, be utilized, such that a sensor 43 will actuate a solenoid valve40, to open the line 37 to the line 38, for opening of the directionalor line valve 31.

With particular reference to FIG. 4, the cylinder or container valve 17is generally illustrated, in threaded engagement at 150 with the neck151 of a cylinder 16.

The valve 17 comprises a casing 152 that has a valve seat 153 fixedlydisposed therein by threaded sleeve member 154 being tightly threaded at155, thereagainst, at 156. The fluid from the container 16 is thereforedisposed within the sleeve member 154, against the piston 157. Thepiston 157 is constructed to have an end-closing portion 158 and agenerally cylindrical portion 160, having a bore 161 therein, andterminating in an extended cylindrical piston portion 162, with asuitable sealing ring 163 sealing the cylindrical portion 160 of thepiston 157, relative to a bore 164 in the housing 152. A small hole 165,allows passage of the incoming fluid from the zone 166 into a cavity 167on the rear side of the piston 157, such that, in the closed conditionof the valve 17 illustrated in FIG. 4, fire extinguishing fluid ispresent in both zones 166 and 167 on opposite sides of the piston 157.The cavity 167 extends to a rearward cavity zone 168 which alsoaccommodates the extended portions 162 of the piston 157, when thepiston 157 is moved in a rightward direction from its positionillustrated in FIG. 4, away from the valve seat 153. Such movement wouldbe against the force exerted by the cylindrically wound compressionspring 170 that tends to urge the piston 157 leftward as viewed in FIG.4, inasmuch as the spring 170 is in abutment at its left end againstsurface 171, and at its right end against surface 172.

Thus, while the port 165 between the front side 166 and rear side 167 ofthe piston 157 is small, it is adapted to allow filling of the cavity167 (including communicably related cavity portion 168) slowly, but notrapidly. It will thus be seen also, that in the position illustrated inFIG. 4 for the piston 157, fluid is prevented from discharge through theline 173 to the line 27. It will be noted that fluid pressure alsocommunicates through the line 174, in the static position for thecylinder valve 17 illustrated in FIG. 4, to a gauge 175, for visualobservation of pressure indication, and also to a burst disc 176. Anormally closed blind plug 179' is provided in the casing 152 foroptional connection to other cylinders, for firing them along withcylinder 16, if desired. The fluid in container 16 is normally a liquid,and in the closed condition of the valve 17, gas from the liquid fillscavity 167. When the valve 17 is open and rapid expansion takes place,the liquid fluid in container 16 is transformed to the gaseous state.

When it is desired to open the valve 17, to permit passage of fluidtherepast, such may be accomplished either by manual actuation of thevalve operator 23, to open or exhaust from the zone 167, by engaging theplunger 300 and leftward movement thereof, as viewed in FIG. 4, todischarge through the port 307, whereby fluid in the cavity 167, 168 maybe evacuated therefrom, thereby reducing pressure on the rearward sideof the piston 157, such that the pressure against the forward surface159 thereof, forces the piston away from the seat 156, or in thealternative, the cavity 167, 168 may be vented to atmosphere through theline 181 by actuation of solenoid valve operator 21. The solenoid valve21 operates in a manner similar to that described above for the solenoidvalve 40, except that it opens the cavity 167, 168 to atmosphere, forrightward movement of the piston 157 away from its seat 153, just asopening of the manual operator 23 so exposes the cavity 167, 168, toatmosphere, for exhaustion of fluid therefrom.

The mechanical valve operator 23 is constructed similar to, and operatessimilar to the mechanical valve operator 38, so duplication thereof willnot be required herein, by way of description and only operator 23 willbe described in detail herein.

The solenoid type operator 21 that evacuates the cavity 167, 168 topermit rightward movement of the piston 157 away from its seat 153 is,itself, operated by means of the sensor 18 that is temperature or smokeresponsive, or responsive to other phenomena, to complete a circuitbetween a power source 20 and ground 19, or in any other manner thatautomatically operates a valve 21 by automatic temperature sensingtechniques.

It is particularly important when the sensing member 18 is of theelectrical type, and when the valve 21 thereof is open to facilitateventing of the cavity 167, 168, that the valve 21 not close should firereach and damage the sensing member 18, or any other part of the wiringsystem or the like that operates the valve 21. To this end, means isprovided for retaining the piston 157 in an open, or rearward retractedposition, away from its seat 153, once either the manual operator 23 orthe electric operator 21 has called for and effected the opening of thevalve 17. Accordingly, a venting mechanism 200 is provided, having aprotruding plunger member 201 that extends into the cavity 168, forengagement by extended piston portion 162. To this end, reference ismade to FIG. 5, wherein the device 200 in the form of an air vent isprovided, that provides a continual bleed of fluid from the cavity 167,168, thereby preventing re-seating of the piston 157. The vent valve 200is provided with a floating plunger member 201 that is engaged by pistonportion 162, particularly by the cammed edge 202 thereof, with the outerdiameter 203 of the cylindrical portion being sized relative to the bore164 at that point, to remain spaced from the bore 164 an amount "A",that may vary depending upon the sizes of the structures, but that issufficient to allow passage of fluid from the cavity 167, 168, outwardlyof the opening 203, when the floating plunger member 201 moves theelastomeric, rubber or the like piston portion 201 and the piston 204that carries it away from the seat 205. It will be noted that thefloating plunger member 201 is provided with a controlled amount of playas indicated by the dimension "B" in FIG. 5, that is pre-determined toopen the piston 204 (and its portion 203) away from the seat 205. Acylindrical wound compression spring 206 is provided, seated at itslower end as viewed in FIG. 5, against a portion 207 of a sleeve-typehousing member 208, that in turn is in threaded engagement at 210 withthe housing 152 of the valve 17. The opposite end of the compressionspring 206 is in engagement against the surface 212 of the piston 204.

The plunger member 201 is captured by overhanding flange portion 213,and is sized for clearance in hole 199 for passage of fluid therebetweenor may optionally be provided with a fluted or longitudinally groovedsurface 214, to provide passageways for fluid being released from thecavity 167, 168, through the passageways 214, through the openingbetween the piston portion 203 and the valve seat 205, through thecavity 215, through the opening 216 between the piston 204 and theannular inwardly protruding ring 217 of the housing 208, and out theopening 218 in a protective cap 220 that in turn is threaded at 221 intoan internally threaded bore 222 in a tool 400. It will thus be apparentthat, once the plunger 214 is depressed by the piston portion 202, thecavity 167 and 168 will remain vented to atmosphere, and the piston 157will remain open for flow of fluid from the zone 166, into the line 173.While fluid will be free to pass into the cavity 167 through the smallport or opening 165, it will be noted that the opening 165 is so smallthat by the time that any significant amount of fluid enters the cavity167, there will have already been complete discharge of fluid from thecontainer 16.

With particular reference to FIG. 3 now, the operator 23 is illustratedas comprising a mechanism adapted to engage the plunger 300 to releasefluid from the cavity 167, 168, through the port 307.

In this regard, the plunger 300 when engaged and moved leftward from theposition therefor illustrated in FIG. 4, will have its left-most endunseated from the seat 305, and be moved against the compression spring306, whereby the fluid in the zone 167 will be free to pass through port312 in plug 301 that in turn is in threaded engagement at 302 in a boss303 carried by the right-most end 304 of the housing 152 of the valve17, with the fluid then passing outwardly of the bore or end opening311, to vent.

The operator 23 has a casing 116, and another casing portion 117 adaptedto be received within the bore or end opening 311 of the valve 17. Thecasing 116 houses a detent piston 118 axially movable in the bore 120 ofthe operator 23. One end of the piston 118 is provided with a threadedconnector 121 that in turn has the cable 26 carried thereby, forgrasping and actuation, in order to move the plunger 118 in thedirection of the cable, for one type of manual actuation thereof. Theopposite end of the piston 118 is provided with a push plunger 25connected thereto by means of bolt 124. Pull pins 125 and 126respectively hold the plunger and pistons against axial movement in thebore 120. Removal of a pull pin 125, 126 allows axial movement of thepiston or plunger respectively. If the pull pin 126 is moved to allowthe cable 122 to actuate the piston, it will be seen that the pushplunger may remain in position, in that the bolt 124, while beingthreaded for movement with the piston 118, is free to slide through thepush plunger until the head of the bolt 124 engages the plunger shoulder127. If both pins 125 and 126 are pulled, either the push plunger or thecable 122 may be utilized to actuate the secondary plunger 128, by thecam surfaces 129 of the piston. The plunger 128 is rightwardly biased asviewed in FIG. 3 by a compression spring 130 engaged against a shoulder111 of the secondary plunger, with the opposite end of the spring 130being in engagement against a surface 110 of a nut 103 that in turn isin threaded engagement with the casing portion 117. It will be notedthat the external surface of the auxiliary plunger 128, at its left-mostend is sufficiently sized relative to the bore 107 through the nut 103,to permit passage of air therethrough after opening of the plunger 310by engagement with member 300, whereby fluid may be vented through thenut 103, through an appropriate port (not shown) in the casing portion117, and out through the vent opening 105.

It will be apparent from the foregoing that any suitable means may beused to mechanically release fluid from, or open the zone 167, toatmosphere, including the use of push plungers, cables, pressure meansthat are responsive to heat or other phenomena in a room, and which maybe operative upon the plunger 300 to open the same, or any othersuitable means.

With particular reference now to FIGS. 5 and 6, a tool 400 is shownhaving a hex portion 401 for wrench engagement and an extension portion402. Portion 402 is internally threaded at 403 to engage the threadedend of member 204 for backing member 204 and consequently the pistonportion 203 away from valve seat 205, for use of the tool 400 tomaintain the device 200 in venting condition, as for example duringstorage or shipment after use, to prevent buildup of pressure fromresidual fluid in a cylinder. In use, the tool 400 allows ventingthrough itself by axial grooves (not shown) through its threaded portion403, through oversize threads, or by any other means, so that ventingmay occur through the hole 218 in its cap 220 when the tool is screwedto a venting position.

It will be apparent from the foregoing that while a particularembodiment of the invention is described and illustrated herein, as wellas particular other features of the environment of use of the presentinvention, the specifics may vary depending upon the desired parametersat any given time, and the details of construction of individual parts,as well as their use and operation may also vary, all within the spiritof the invention as defined in the appended claims.

What is claimed is:
 1. In a valve for pressurized fluid, of a typecomprising:(a) a casing having an inlet for the pressurized fluid at oneend thereof and having an outlet including a valve seat at the otherend; (b) a piston positioned within said casing having a wall definingan internal cavity in continuous communication with said casing inletand having a closed end, said piston being slidable within said casingbetween a first position wherein said closed end engages said valve seatand a second position wherein said closed end is displaced from saidvalve seat; (i) said piston having at least one opening through saidwall adjacent said closed end thereof, said opening providing acommunication path between said internal cavity and said outlet whensaid closed end is displaced from said valve seat, whereof said pistonhas a first pressure-operative surface portion exposed to fluid pressurefrom said internal cavity so as to urge said piston toward said secondposition and a second pressure-opertive surface portion normally not indirect communication with said internal cavity; and (c) separate,selectively-open, pressurized fluid path means for connecting said inletto said second pressure-operative surface portion for selectively urgingsaid piston toward its second position for displacing said closed endfrom said valve seat; the improvement including means for venting saidsecond pressure-operative surface to said internal cavity only when saidpiston is in said second position whereby said piston remains in saidsecond position.
 2. The valve of claim 1 wherein said venting meansincludes a vent passageway through said piston wall connecting saidinternal cavity with said second pressure-operative surface portion,said connection being made only when said piston is in said secondposition.